Liquid ion chamber (LIC) imagers are routinely used as either an alternative to or a complement of traditional x-ray imagers using radiographic film. In a current LIC configuration, the imager is comprised of 256 electrode rows by 256 electrode columns, thus giving a total of 65,536 imaging sites. Each set of electrode strips are incorporated on a printed circuit board (PCB) and the two PCB's are separated by a distance of approximately 1 millimeter. The volume between the two PCB's contains the ionization liquid, typically 2,2,4 trimethylpentane.
In operation, a high-voltage switching system is used to sequentially apply a 20 millisecond high voltage pulse to the electrode rows. During each pulse, the ionization currents from the 256 electrode columns are simultaneously measured. In this configuration, to obtain sufficient ionization currents a polarizing voltage between 300 and 500 volts is required. With a 20 millisecond pulse length, a complete image is acquired in approximately 5 seconds. Since the patient is exposed to radiation during image collection, the shorter the image time the lower the dose of radiation received by the patient. However, the image time can only be reduced by decreasing the high voltage pulse length to the electrodes and by designing an electrode structure that can give sufficient output over this reduced time period.
FIG. 1 is a cross-sectional view of a small area of a LIC matrix. An electrode 1 is the high voltage electrode while electrodes 2 are the ionization current collecting electrodes which connect to the measurement apparatus. An ionization liquid 3 separates electrode 1 from electrodes 2. In this configuration areas 4 and 5 would be considered individual ionization chambers. However since only distance separates the individual chambers, not a physical barrier, there is nothing to prevent ions from being collected by adjoining ionization chambers.
In operation, the high energy x-rays used during the imaging process cause the creation of positive and negative ions within ionization liquid 3. The application of a polarizing voltage cause the ions to move to electrodes 1 and 2, i.e., to an electrode with an opposite sign from that of the ion. A current pulse is formed due to the ions being neutralized at the surfaces of the electrodes. The current pulses are then amplified and used to determine the ionization in that region. Therefore, as a result of the voltage field, the ions are gradually swept out of the liquid.
The period of time that it takes to sweep the ions out of the liquid (i.e., neutralize the ions) is called the transit time. The transit time, T, is equivalent to the distance, d, between electrodes divided by the ion drift velocity. Drift velocity is approximately equivalent to the ion mobility, ,u, times the electric field, E. The transit time is given by: EQU T.apprxeq.d/[uE] (1)
The lower the electric field or the greater the distance between the electrodes, the slower the ion velocity and the longer the transit time. Long transit times lead to slow image acquisition as well as increased radiation dosage to the patient.
Increasing the electric field decreases the transit time while increasing the number of ions captured per pulse. Unfortunately increasing the voltage also increases the safety risks associated with the imager. Furthermore, the high voltage switching circuits required to sequentially pulse the high voltage electrodes tend to have a short lifetime which is compounded by exposure to radiation. This high failure rate leads to LIC instrument unreliability and high cost. A means of increasing the electric field without increasing the voltage applied to the electrodes is therefore desirable.
The field between the electrodes can be considered in two pans. First, there is the uniform field between the parallel pans of the electrodes and second, there is a fringe field associated with the edges of the electrodes. For the uniform part of the field the electric field, E, is linearly related to the voltage, V, applied to the electrodes and their spacing, d, as: EQU E=V/d (2)
The fringe field is not uniform and resembles the field of an isolated point of charge. This field decreases inversely with distance so that it is high close to the electrode, decreasing with distance from the electrode. For either component of the electric field, the highest field for any applied voltage is achieved using a small spatial distance.
From the foregoing, it is apparent that a LIC electrode assembly which offers small transit times, high efficiency ion collection, and low voltage operation is desired.